1. Field of the Invention
The present invention relates to an intake control system for an internal combustion engine, which sets a target intake air amount according to a target torque, to thereby control an intake air amount.
2. Description of the Related Art
A conventional intake control system for an internal combustion engine is disclosed e.g. in Japanese Patent Publication No. 4832542. The intake control system sets a target intake air amount according to a target torque of the engine in the following manner: First, a provisional target intake air amount, which is a provisional value of the target intake air amount, is set to an initial value. Then, an estimated retard amount of ignition timing, which is estimated to be set assuming that the provisional target intake air amount of intake air is supplied to the engine, is calculated according to operating conditions of the engine at the time. Next, under these conditions of the provisional target intake air amount and the estimated retard amount, a torque estimated to be output from the engine is calculated as an estimated torque.
When the difference between the calculated estimated torque and the target torque is large, the provisional target intake air amount is corrected such that the estimated torque becomes close to the target torque. Specifically, when the estimated torque is larger than the target torque, the provisional target intake air amount is reduced, whereas when the opposite is the case, the provisional target intake air amount is increased. The estimated retard amount and the estimated torque are calculated again based on the corrected provisional target intake air amount. Such a process is repeatedly performed until the estimated torque converges to the target torque. When the estimated torque has converged to the target torque, the provisional target intake air amount at the time is set as the target intake air amount that attains the target torque. On the other hand, when the estimated torque does not converge to the target torque in spite of execution of the above-described process a predetermined number of times, the provisional target intake air amount at the time is set as the target intake air amount.
When the air-fuel ratio of a mixture burned in the engine is controlled to a fixed value, e.g. to a stoichiometric air-fuel ratio, normally, the torque of the engine has a characteristic that it increases as the intake air amount increases. The above-described conventional intake control system is preconditioned on such an output characteristic of torque. However, for example, in a case where the engine is a high compression ratio engine, and to suppress knocking, the retard amount of the ignition timing is set to a relatively large value, particularly when an engine coolant temperature is high or when an intake air temperature is high, the combustion efficiency of the engine is reduced by retarding of the ignition timing, so that even when the intake air amount is increased, the torque of the engine is not increased or even reduced. In this case, for one target torque, there are a plurality of solutions to the intake air amount for attaining the target torque.
However, the conventional intake control system employs a method of finding an intake air amount that attains a target torque, by using an initial value of a provisional target intake air amount to start with, and thereafter repeatedly performing correction of the provisional target intake air amount and checking of a state of convergence of an estimated torque based on the corrected provisional target intake air amount to the target torque. Therefore, there is a possibility of selecting a larger solution to the intake air amount from the above-mentioned plurality of solutions depending e.g. on a setting of the initial value of the provisional target intake air amount. In this case, according to selection of a larger intake air amount than required, an excessive amount of fuel which does not contribute to an increase in the torque is consumed, causing degradation of fuel economy. Further, for example, when the provisional target intake air amount is set to a larger value than the larger solution to the intake air amount, hunting of correction of the provisional target intake air amount is caused e.g. by correcting the provisional target intake air amount to a still larger value, which prevents the estimated torque from excellently converging to the target torque, and hence in this case as well, there is a fear that it is impossible to properly set the target intake air amount.